A polyphenol is a chemical compound characterized by the presence of more than one phenol unit or building block per molecule, i.e. an alcohol containing two or more benzene rings that each has at least one hydroxyl group (OH) attached. Polyphenols are generally divided into hydrolyzable tannins (gallic acid esters of glucose and other sugars) and phenylpropanoids, such as lignins, flavonoids, and condensed tannins.
As described in K. A. Leiper et al., J. Inst. Brew. 2005, 111(2), 118-127, colloidal instability in beer is caused mainly by interactions between polypeptides and polyphenols. These combine to produce a visible haze following storage, particularly if there is oxygen in the beer as this causes the polyphenols to oxidise resulting them becoming more haze active as their affinity for haze active proteins increases. Haze is not only visually unappealing but detrimental to the taste of the beer and reduces the beer's shelf life. Reducing the levels of one or both of polypeptides and polyphenols using suitable stabilising treatments will extend physical stability.
Polypeptides responsible for haze formation in beer originate mainly from barley, range in size from 10 kD to 30 kD and are rich in the amino acids proline and glutamic acid. They are heavily glycosylated with glucose and account for only 3-7% of total beer protein.
Polyphenols in beer originate from barley and hops. Beer contains approximately 100-300 mg/L polyphenol and these can be divided into derivatives of hydrobenzoic and hydroxycinnamic acids and flavanols and their derivatives. The latter group account for 10% of total beer polyphenols and contain species related to colloidal instability. Tannoids are polyphenols that are oxidised to form tannins, and these are known to combine with the molecule polyvinylpyrrolidone (PVP) to cause haze.
Flavanoids (oligomers of flavanols) all have the same basic structure of two aromatic rings linked by a three carbon unit and they are often hydroxylated to varying degrees and these groups are sometimes glycosylated or methylated. Flavanols found in beer are catechin, epicatechin, gallocatechin and epigallocatechin. These can exist as monomers but are more commonly joined to form flavanoids as dimers, trimers or larger polymers.
Prior methods for removing haze-active proteins involved the use of non-selective enzymes: these would remove most of the protein from the beer, resulting in beer that did not foam. Agents were then added to the beer to allow it to foam. More recently enzymes that target proline rich proteins have become available. The most successful of these is Clarex™, a product available from DSM Food Specialties. The enzymes are safe and do not prevent the formation of foam.
The addition of tannins to beer that form complexes with proteins in beer is a well established technique. The tannins are comprised of oxidised tannoids and therefore readily form an insoluble precipitate (haze) that can be filtered from the beer.
However both the tannin and enzyme dosage methods have a major drawback; because they are soluble in beer they may fail to comply with legislation regarding additives in beer. In addition, there is considerable consumer pressure in Europe to remove additives from beer and other beverages. It would therefore be desirable to devise a method of preventing haze in beverages without using soluble adsorbents.
Stabilisation of beer using insoluble adsorbents that are insoluble and selective for haze active proteins has resulted in only one feasible solution: silica gel. The silica gels used in beer stabilisation have a very high surface area and numerous selective binding sites for haze active proteins. They are highly effective. Silica gels are widely available as both hydrated and non-hydrated products. The hydrated products are easier to handle as powders, the non-hydrated gels are more effective. The key drawback of using silica gels is that they are not practically regenerable in a brewery.
Polyvinylpolypyrrolidone (also known as PVPP, cross-linked polyvinyl pyrrolidone, crospovidone or crospolividone) is an insoluble, highly cross-linked form of polyvinyl pyrrolidone (PVP). PVPP compositions are commercially available: examples of commercial compositions include the range of PVPP products sold by ISP Corporation as Polyclar®.
The use of polyvinylpolypyrrolidone to remove polyphenols in beer production and thereby stabilise the beer is known in the art. Prior art PVPP formulations are typically in powder or agglomerate form. For example, U.S. Pat. No. 5,484,620 describes polymer blends including PVPP and polyethylene, and their potential use in filtering or clarifying beer to adsorb haze-causing impurities such as polyphenols. The polymers may be sintered together to form an agglomerate.
U.S. Pat. No. 7,767,125 describes a polymer blend comprised of polystyrene and water-insoluble cross-linked PVP. These polymer blends are stated to be suitable as filter aids for filtering alcoholic beverages, such as wine or beer, and non-alcoholic beverages, such as fruit juice. The polymers are blended and the mixture extruded and pelletized.
However, a number of disadvantages are associated with the use of the known powder, pellet and agglomerate formulations. In particular, there are a number of safety risks (in particular relating to inhalation) associated with the handling of loose powder formulations. The powder formulations also readily form a cake on wetting: this can cause fouling of the beverage stream being passed through it and takes more time and effort to remove and replace in preparation for the next batch.
M. Rehmanji et al., MBAA TQ, 2002, 39(1), 24-28, describes stabilisation of beer with a composite of micronised PVPP and carrageenan (Polyclar® Brewbrite™). The product is stated to reduce the level of the haze active tannoid fraction.
The addition of (non-cross-linked) PVP to membranes, such that the PVP acts as wetting agent, is known in the art. However, membranes manufactured from materials such as polyethersulfone and poly(vinylidene fluoride) are too hydrophobic to be used effectively for filtering beverages without the incorporation of wetting agents. These wetting agents can leach into the product and there are therefore limits on the amount that can be used.
U.S. Pat. No. 7,172,075 describes a method for manufacturing such a membrane; this membrane includes a barrier layer which contains PVPP immobilised in a polymer matrix. In this membrane application, the PVPP is part of the membrane's formulation in order to give it the appropriate physical properties, namely to increase porosity. However, the membrane formulation contains insufficient PVPP to enable it to act as a sorbent material for large scale beverage stabilisation applications: the total mass of PVPP present in the membrane would only provide a dose of a few milligrams per hectolitre of beverage in use. Consequently, if used in large scale beverage stabilisation applications, the PVPP would be rapidly exhausted after a few litres of beverage had passed through and the membrane subsequently incapable of removing further polyphenols.
U.S. Pat. No. 4,798,847 describes the preparation of hydrophobic membranes including a hydrophobic polymer and a hydrophilic polymer in a suitable solvent medium. The hydrophilic polymer may be polyvinyl pyrrolidone, which may be cross-linked. However, in this publication the polyvinyl pyrrolidone chains are cross-linked to the substrate, rather than to themselves. Moreover, the hydrophobic polymer used for the membranes in this publication is polyether sulfone or polyether imide: these polymers are expensive to produce and therefore impractical for large-scale beverage stabilisation applications. In addition, the small total mass of PVPP present in the membrane would also be rapidly exhausted after a few litres of beverage had passed through and the membrane subsequently incapable of removing further polyphenols, thus making such membranes unsuitable for large scale beverage stabilisation applications.
WO 02/34375 describes composite filters and methods for preparing them. However, these do not use PVPP: instead, these use non-cross-linked PVP as a wetting agent.
US 2008/0169234 describes a roll type product that is comprised of cellulose filter sheet media. However, the material used to form this product is generally elastic. Elasticity is disfavoured for filtration materials for beverage applications, as using such materials would cause a rise in the pressure drop during use. A high pressure drop requires much greater energy costs to achieve flow, and limits the application of the method to high pressure vessels and ancillary equipment.